The present invention relates generally to plug valve assemblies, and more specifically, to plug valve assemblies employing polymerically encapsulated metal valve seats and to a method of making polymerically encapsulated metal valve seats for plug valve assemblies.
Users of valves have long desired a valve which is capable of satisfactory sealability at higher pressure drops and temperatures. Higher pressure drops place more stress upon the seals or seats inside the valve. As a result, the valve seals begin to leak or fail completely (a condition known as xe2x80x9cblow-outxe2x80x9d). To remedy such problems, current higher pressure operations often use plug-type valves.
Many plug valves are adjustable and, thus, can compensate for leakage. To provide greater sealability, most plug valves employ xe2x80x9csoftxe2x80x9d or plastic seats. Soft seats allow for greater sealability between the seat and the plug, thereby preventing leakage. However, soft seats, while providing a greater degree of sealability, increase the opportunity for blow-out. Due to the lower rigidity of soft seats, they commonly suffer blow-out at higher or throttling pressures.
To prevent blow-out, conventional soft seats are commonly reinforced by a pair of spines or ribs extending outwardly from the chamber wall on either side of the flow channel. These spines or ribs hinder the pressure created by the fluid passing through the flow channel from pushing the soft seats out of position and causing blow-out. However, inclusion of spines or projections from the chamber wall prevents the chamber wall from being easily machined, leaving the chamber wall with a rough as-cast surface.
U.S. Pat. No. 3,360,236 to Huslander discloses a plug valve seat. The seat in Huslander comprises a thin flexible core of a material such as steel with a sealing material, such as polyurethane, bonded thereto. The seat fits into grooves on the plug and rotates with the plug rather than remaining in a fixed position. However, it appears from the figures that the seat in Huslander is not encapsulated in its entirety. Rather, only the edges have been coated with a plastic material. Further, the seat is thin and flexible, providing little or no support against blow-out. Additionally, the seat rotates with the plug in pair of grooves in the plug surface. Grooves in a machined surface, such as the plug in a valve, are difficult and exceedingly expensive to machine.
U.S. Pat. No. 3,326,519 to Freed solves some of those problems mentioned above by providing an encapsulated metal reinforced valve seat. However, Freed uses a perforated metal sheet as a reinforcing material for the valve seat. A perforated sheet used as a reinforcing material for a valve seat allows the valve seat to deform when exposed to high pressures because the perforations weaken the structural integrity of the seat.
Accordingly, a need still exists for a plug valve and a seat for a plug valve which substantially reduces the required turning torque of the valve while still providing superior sealability, and protection against leakage and blow-out of the valve seats. A further need exists for a method to make such a valve seat.
The present invention meets these needs by providing an improved plug valve assembly having reduced turning torque with superior sealability and protection against seat blow-out. The plug valve assembly of the present invention substantially reduces the required turning torque while maintaining sealability by providing a chamber wall capable of being machined to a smooth surface. Turning torque is also reduced by eliminating the sleeve liner insert favored by the prior art.
In accordance with the present invention, a plug valve assembly is provided. The plug valve assembly includes a valve body having an internal chamber, a plug disposed in that internal chamber and seat members for supporting the plug. The internal chamber is defined by an inner wall. Preferably, the inner wall has a substantially smooth, machined surface, allowing reduced turning torque and greater sealability. Most preferably, the machined surface has a smoothness value of about 125 rms to about 1 rms.
The seat members function to both support the plug and to seal the valve. The seat members preferably comprise a rigid, solid support frame encapsulated in a polymeric material, but may also be formed of a metal or ceramic material. By xe2x80x9csolidxe2x80x9d it is meant that the support frame is essentially non-perforate. However, the support frame may be solid and still have grooves, indents, notches, and other types of locking channels or areas of reduced thickness therein. In the preferred seat members, the support frame is preferably a solid metal frame and the polymer is preferably a melt-processable polymer, such as a fluoropolymer.
The seat members form a sealing surface with both the inner wall of the internal chamber and the plug. The sealing surface with the plug may have a vertical taper of from about 1xc2x0 to about 8xc2x0. The plug then has a vertical taper corresponding to the vertical taper of the sealing surface. In addition, the sealing surface with the inner wall may also have a vertical taper on the order of 1xc2x0 or 8xc2x0 with the inner wall having a corresponding vertical taper.
The valve assembly may further include locking members disposed in the chamber for preventing substantial horizontal, vertical or rotational movement of the seat members. These locking members fix the seat members in location in the present invention. Ideally, the locking members are depressions formed in the inner wall of the chamber. The plug valve assembly further includes an adjustment mechanism. The adjustment mechanism adjusts the plug vertically within the seat members to prevent leakage.
The method of making the encapsulated valve seats of the present invention is believed to be particularly unique. The encapsulated valve seats of this invention are formed by injection molding using a mold which has a means for positioning the support frame in the mold. The mold cavity includes at least three retractable pins which contact the support frame to position it in the center of the mold cavity during the injection of the polymeric material so that the support frame receives an even coverage of polymeric material on each side.
The method for encapsulating the valve seat includes the steps of providing a mold including a first plate having a cavity therein and a second plate having a cavity therein, the cavities forming an opening such that when the plates are brought together the opening has a shape of an encapsulated valve seat, one of the plates having an injection sprue therein; positioning a support frame in the mold so that the support frame is located in the center of the opening when the plates are brought together; bringing the plates together to close the mold and form the opening; heating the mold; injecting a heated, fluent polymeric material through the sprue into the opening to encapsulate the support frame in the polymeric material; cooling the mold and the polymeric material to solidify the polymeric material to form an encapsulated valve seat; and removing the encapsulated valve seat from the mold.
Accordingly, it is an object of the present invention to provide an improved plug valve assembly with reduced turning torque while maintaining superior sealability and protection against blow-out. It is a feature of the present invention to provide a plug valve assembly having seat members comprising a rigid support frame encapsulated in a polymeric material. It is a further feature of the present invention to provide a method for forming an encapsulated seat member for a plug valve in which the seat member comprises a rigid, solid support frame encapsulated in a polymeric material. Other objects, features and advantages of the present invention will be apparent from the following description, the accompanying drawings and the claims.